1. Field of the Invention
The present invention relates to grinding wheels and more specifically to grinding wheels to remove the sprues and riser pads from the casting process for railroad car wheels. The railroad wheels generally are cast in a graphite mold, which includes sprues and risers for the molten metal. However, sprues and riser pads represent extraneous metal on the product after the casting process, which metal must be removed before further wheel processing.
The preferred method for manufacturing cast steel railroad wheels is the bottom pressure casting foundry operation wherein molten steel under pressure is forced upwardly into a graphite mold and filled from the bottom upwardly. Bottom-pouring pressure casting operations eliminate many of the concerns associated with the more traditional top-pouring technique for molten steel in foundry operation. These concerns include metal splashing and insufficient filling of molds. In the casting of railroad wheels, the front side of the wheel, which also corresponds with the top half of the mold, typically has a raised center hub portion. There are generally from 9 to 14 raised sections or risers extending from the front plate portion of the wheel near the hub, which number depends on the size of the wheel.
Risers are designed to hold additional metal to fill downwardly into the mold during the cooling and solidification of the wheel just after pouring. The raised sprues extending from the plate are remnants of the risers. The sprues are difficult to remove and would require considerable effort if removed by hand-held grinders. As a consequence, hand-held grinding operations are not generally used in present wheel-making operations. One method for removal of cast railroad-wheel sprues is a so-called sprue washing operation, which is a carbon-arc melting of the raised sprue. In this technique, a hollow electrode melts the sprue and air blown through the hollow portion of the electrode blows away the molten metal. However, the molten metal from the sprue washing operation is deposited on adjacent sections of the wheel, which requires a subsequent time-consuming and awkward chipping process for removal of the spattered metal. Further, the sprue washing operation requires the operators to wear a protective suit with a separate airhood supply and adequate noise protection.
After completion of the sprue washing and chipping operations, the center raised hub section is removed during the flame cutting of the axle hub, which is later finished by a hub-boring operation. The cast steel wheel is then heat treated, cleaned by a shot-blast operation, and sprue-removed surface areas are finish ground. The finish grinding is typically a manual operation.
Machine grinding of ingots and billets is known in the steel industry. These operations typically scarf a cooled ingot surface to remove minor cracks or surface imperfections, although certain scarfing operations are preferred when the ingot is at an elevated temperature. An apparatus for the automatic grinding removal of sprues from a cast steel railroad wheel shortly after the wheel has been cast and solidified is taught in U.S. Pat. No. 5,209,021 to Christie et al.
In the Christie et al.--'021 patent, the sprues from cast steel railroad wheels are removed when the wheel has cooled from initial casting to a temperature of from 800.degree.-1,200.degree. F. (425.degree.-650.degree. C.). The grinding is accomplished by a heavy grinding wheel or stone approximately 25-inches in diameter and 3-inches thick (63 cm.times.7.6 cm). The grinding wheel is mounted on a spindle and arbor, and driven by a relatively large electric motor. Removal of sprues by the grinding operation and the finish grinding of the wheel to the final contour in the sprue areas can be done in a single operation with a single grinding wheel.
Almost all railroad wheels broadly have a shape with an inner hub portion, an outer tread portion including a flange, and a relatively thin metal web or plate connecting these inner and outer components. This web or connecting plate usually incorporates a curved contour for the promotion of strength, heat transfer or other physical characteristics associated with the intended wheel service application. The hub portion has an outer or front face, an inner or back face, and an axle through bore, which extends therebetween along the axle axis.
The tread portion of the wheel also has an outer or front face and an inner or back face with the flange radially extending from the back face as an annulus, and the tread tapered from the front face radially outwardly toward the flange.
Railroad wheels can be between 28 and 42 inches in diameter; the hub may have a diameter between about 10 and 13 inches; and, the connecting plate or web is contoured with either a concave or convex shape. Consequently, it can be appreciated that the sprue and riser pads on the contoured connecting plate may not be readily accessible for grinding, especially on smaller diameter railroad wheels where the riser pads are in close proximity to the protruding wheel hub. In addition, any mechanical secondary operations are expressly prohibited from negatively impacting the strength and integrity of the railroad wheel, as safe railroad wheel operation is a primary concern. No undue risks can be taken to compromise the structural integrity and performance of these railroad wheels, since wheel failure has the potential of producing a catastrophic event.
2. Prior Art
Railroad wheels are generally cast in graphite molds from either molten steel or iron. This casting practice has evolved over nearly a century of progress. Casting processes generally involve secondary mechanical practices to remove extraneous material or to true the casting to dimensional tolerances. These secondary operations are undesirable, as they are costly and time-consuming. However, removal of the extraneous material in finishing metal operations is necessary to provide products that are both functionally and aesthetically acceptable. Metal finishing operations include machining, flame cutting or torching and grinding. Hand grinding is used for some semi-finished goods, such as billets in a steel mill. However, for products that are to be transferred to a finishing line for shipment more careful and sophisticated tools are often used, such as grinding machines.
In the production of railroad wheels, it has been the practice to mount an as-cast wheel in a swing-frame grinding machine, and thereafter to carefully grind the sprue and riser pad. However, it had been found that the present grinding machine and grinding wheel apparatus cannot adequately access the railroad-wheel web to remove the entire riser pad areas, which are in close proximity to the hub. Further, it has been found that the presently available grinding wheels with square edges or corners could potentially mar the web surface at initial use of such grinding wheel, which could result in a rejected wheel product.
In the railroad industry, the railroad wheel sizes are generally large with diameters between about 28 and 42 inches and a weight in excess of 500 pounds. Thus, an apparatus to accommodate the secondary working of such large products is required to be large. However, the secondary operations themselves may be subtle and relatively delicate. For example, finish grinding of the bearing surface on a gear may have a tolerance of only a few ten thousandths of an inch. Alternatively, the bulk removal of extraneous material requires the placement of a relatively large diameter grinding wheel against a large surface in a very inconspicuous location to perform a finishing operation in a short time. Performance of a finishing operation in a short time relates to both operator time and the economic cost from work in process.
As noted above, railroad wheels are frequently finished by grinding operations, but as railroad wheels are large mass pieces, the equipment to hold and grind the railroad wheel is appropiately sized to accommodate this large mass. The grinding operation is performed by a grinding wheel on a spindle, as known in the art. The grinding wheel is a compacted abrasive material in an annular shape with a fixed diameter and wall thickness, which usually rotates in the same direction as the product to be ground. The compacted abrasive material of the grinding wheels may have been bonded by an alloying process in a furnace or by use of an adhesive type bond. Grinding wheel shapes are often disks formed with flat radial faces and outer radius surfaces, or wall thicknesses, perpendicular to the radial face. As a consequence, the intersection of these radial and outer radius faces produces sharp corners, which when used to grind railroad wheels can result in a sharp angle of attack on the to-be-ground surface, until natural wear and erosion reduce the severity of the angle at such corner.